THE DISINTEGRATED SURFACES OF CRYSTALS. \Qg 



face is considerable, the optical figure loses its symmetry, and gradually passes into 

 other figures produced by the other faces of the crystal towards which the arti- 

 ficial face is inclined. 



On some occasions I have found the principal radiations united by a beautiful 

 nebulous web of a triangular form (as shewn in Fig. 9). 



All the figures above described, may be seen by reflection from the smallest 

 portion of the face of the octahedron, and they are often more beautiful on one 

 part than another. The principal radiations are shewn in the figures as if they 

 were seen from the centre of the triangular face, in which case they point to the 

 angles ; but in all other cases, the radiations are perpendicular to the opposite 

 sides of the triangle. 



If we expose any of the six square faces perpendicular to the three axes of 

 the octohedron to the action of water in the manner already described, and exa- 

 mine the optical figure which it produces by reflection, we shall see four rectan- 

 gular radiations as in Fig. 10, each radiation being perpendicular to a side of the 

 square, and consequently passing into one of the three radiations formed by each 

 face of the primitive octohedron. By successive actions, these fom^ radiations 

 become shorter towards the central image, which gradually grows fainter and 

 sometimes disappears. 



If the same experiment is made with the twelve faces formed on the twelve 

 edges of the octohedron, we shall obtain a figure Avith two radiations, forming an 

 oval line with the image of the candle in the middle of it. This image becomes gra- 

 dually nebulous and finally disappears, leaving a kind of elongated oval nebula, 

 with a dark oval centre, as shewn in Fig. 11, where the line AB is perpendicular 

 to the replaced edge, and parallel to an axis of the octahedron. The two radia- 

 tions A, B, obviously pass into one of the three radiations given by the adjacent 

 faces of the octohedron ; and if we were to cut a great number of artificial faces 

 variously inclined from that which gives the two radiations in Fig. 11, to that 

 which gives the fou?- in Fig. 10, we should observe Fig. 11. gradually passing into 

 Fig. 7, and acquu-ing a third radiation, and Fig. 7. passing into Fig. 10, and ac- 

 quiring a fourth radiation. 



From the phenomena exhibited by alum, I proceed to those produced by 

 Jluor-spar, a mineral having the same primitive form and cleavage. Having im- 

 mersed one of the faces of the octohedron for a few days in sulphuric acid, I ob- 

 tained by reflection the beautiful figure shewn in Fig. 12. The three principal 

 radiations A, B, C, with the luminous triangular centre, are first developed, and by 

 continuing the action of the acid, six new images are produced at ef, g h, and i k, 

 connected by lines of light with the other part of the figure. A continuance of 

 the action developes six luminous curves proceeding from the images ef, gh, i k, 

 as in Fig. 13, having each a new image within their concavity. Three insulated 



VOL. XIV. PART I. Y 



